If you're not going to run a return line, then just plug it and use the vented cap. As for the sending unit, I'd replace it with whatever will work considering your future plans.

 

i know it may be too big for my engine
but i just picked up a practically new edelbrock performer 750 for a whopping nothing and the car is running great
now i just need to get some good plugs and wires and set timing

 

finally tracked down the person that built the engine and it does ahve vortec heads, it also has been balanced and blue printed
i just wished he could find the spec sheets if he has any